Gas sensors of similar kind are described for example in German Published Patent Application No. 198 15 700. Such a gas sensor has a potentiometrically driven electrochemical cell including a first and a second electrode and a solid electrolyte arranged between the first and second electrodes. The first electrode is coated with a porous protective film and is in contact with a measuring gas located outside the sensor element. The second electrode is arranged in a reference gas space, which is at least partly filled with a porous material. A heater is be provided for the purpose of heating the sensor element in a measurement area, and is separated from the solid electrolyte coatings that surround it by a heater insulation.
If differing oxygen partial pressures arise in the measuring gas outside the sensor element and the reference gas in the reference gas space, a Nernst voltage is formed between the first and the second electrode, which may be calculated using electronic evaluation means located outside the sensor element. The Nernst voltage may be used to determine the ratio of the oxygen partial pressures in the measuring gas and the reference gas.
Moreover, the electronic evaluation means creates an electrical pumping voltage between the first and the second electrode, which causes oxygen to be pumped into the reference gas space via the first and the second electrodes. As a consequence, there is always adequate oxygen partial pressure in the reference gas space, regardless of the operating conditions. An alternating voltage is also applied between the first and the second electrodes to regulate the heater. The electronic evaluation arrangement may be used to calculate the temperature of the sensor element's measurement area from the temperature-dependent impedance, so that the heater may be switched on or off.
The disadvantage of the conventional gas sensor is that the generation of a voltage serving to pump the reference gas space at the potentiometrically driven electrochemical cell causes the probe signal to be distorted by polarization effects, particularly at the first electrode. The polarization effects are stronger at low probe temperatures, which are present particularly in the case of a gas sensor arranged on the gas outlet side of a catalytic converter.